Nothing really. Perhaps it came from some earlier expression which has lost a few words along the way. The two words put together in that way make up the idiom, so it is not useful to try to break it down. It is a bit like:
- rhoi'r fidl yn y to - to give up
You can translate that as 'putting the fiddle in the roof', but that doesn't help you very much in understanding the idiom.
- Sa i'n (gwybod, mynd, prynu...) - I don't (know, go, buy...)
That is a very common, informal way of saying 'I don't...' in most of south and south-east Wales, as is sana i'n... in parts of west Wales - they both come from different contractions of different sets of words that are no longer used in the same way. Different people will offer different explanations of how they came about, but all we need to understand nowadays is that they mean 'I don't...'.